Field of the Invention
The invention relates to a configuration for ETR (electrothermal) print head triggering, with memory means and a control for an ETR printing unit, wherein energy from an energy source for various pixels of a printed image are furnished to the electrodes of the ETR printing unit. An ETR printer can be used in a postage meter, for instance, for franking mail.
An ETR printer includes not only mechanics but also an electronic head control, an ETR print head with a number of electrodes, and a current collector electrode, which are connected to an energy supply. The printing energy is fed as a constant current into each current path belonging to each electrode, to assure uniform print quality.
The ETR print head acts upon the recording medium, preferably paper, through a resistance ink ribbon moved along with the recording medium. The resistance ink ribbon has an upper resistor layer which is in contact with the ETR print head, a middle current return layer, and a lower ink layer that is in contact with the recording medium.
The ETR print head includes a number of electrodes that are disposed in such a way that they are insulated from one another, and each of which can generate one pixel of the printed image. The energy delivered through the electrodes is converted, in the region of the resistor layer assigned to each pixel, into electrical heat that leads to melting of the ink of the ink layer located in that region.
Published European Application No. 0 301 891 A1, corresponding to U.S. Pat. No. 5,005,993, discloses such an ETR printer with return electrodes. The energy to be delivered is dependent on the resistance of each current path assigned to a pixel, on the melting temperature of the ink, on the intended contrast of the printed image, and on the speed of the moving resistor ink ribbon, and rises non-linearly with the roughness of the surface of the paper.
German Published, Non-Prosecuted Application 38 33 746 A1 has already disclosed a switch unit being acted upon by a trigger unit, for a print head, which unlike the ETR print head, already contains the resistor elements themselves (thermotransfer printing) and has selective triggering with preheating of the resistor element to reduce the heating output in printing.
A serial/parallel shift register acted upon by the serial printing data passes the printing data in a first triggering phase to the latches of a buffer memory or store. In a second triggering phase, during a strobe pulse, each gate triggered by the associated outputs of the latches is switched open, and a trigger pulse is output to the applicable resistor element. The resistor heating elements are preheated directly, by means of a clock frequency that is adapted in both pulse height and pulse width to the necessary heating energy.
In an ETR printer, such preheating by energy from a voltage source is impossible in principle, because the resistor elements are located in the resistor layer of the resistor ink ribbon.
Since a very great number of parasitic serial resistances of variable value (junction resistance between the electrode and the ribbon, track resistance of the layer of aluminum in the ribbon, junction resistance between the ribbon and the return electrode) occur in the overall system including the ETR head with the electrodes, the ETR ribbon and the return electrode, which lead to a variation in the total resistance during operation, an energy supply by means of a voltage source is not suitable, since the varying partial voltage through the heating (printing) resistor would lead to varying printing energies. The result would be fluctuating print quality.
Energy supply to the various electrodes of an ETR head is best done, from a technical standpoint, by means of a constant current source, because a very uniform printing output can be guaranteed as a result of the accuracy of the constant current and of the specific ribbon resistance.
However, a technologically optimal construction with current regulation for each electrode path is often unsupportable in price, because of the (sometimes) very high numbers of the electrodes in an ETR head.
Structures are already known with which the attempt has been made to achieve a technologically feasible construction at acceptable expense. They includes the method of integrating a dropping, protective or multiplier resistor into each electrode path, having a resistance which is dimensioned as approximately 3 to 4 times higher than the effective heating (printing) resistance of the ETR ribbon.
Due to such an artificially increased total resistance of the system, the narrow relatively slight changes in the parasitic serial resistances in the system cannot cause any substantial change in the effective voltage across the heating resistor. In that way, the current of each electrode path has been "stabilized", and an improvement in print quality is attained as a function of a ratio between the dropping, protective or multiplier resistors and the effective heating resistance of the ETR ribbon.
Although that structure is inexpensive and technologically simple on one hand, nevertheless on the other hand it has the considerable disadvantage of needing only a fraction of the energy fed into the complete system for the actual printing process. The great majority of the energy is converted into lost heat. Moreover, a fluctuation in the voltage across the applicable heating resistor is unavoidable, because in contrast to the principle of thermal transfer printing, in the ETR printing principle, during the motion of the ribbon, varying junction resistances at the contact points of the resistor layer of the resistor ink ribbon with the electrodes of the ETR print head and of the current collector electrode, as well as varying resistor heating elements in the ribbon, are operative during the motion of the ribbon.